1. Field of the Invention
The present invention concerns coating compositions. More particularly, the present invention concerns electroconductive sprayable primers. Even more particularly, the present invention concerns substantially 100% solids, sprayable, radiation curable electroconductive coating compositions.
2. Prior Art
As is known to those skilled in the art to which the present invention pertains, many large, durable and lightweight plastic compositions are formulated from what is known as sheet molded compound (SMC). Sheet molded compound is, essentially, a thermoset resin mixture comprising a polymer, glass fibers, and fillers. SMC is noted for its lightweight, durable and consistent part dimensional stability i.e., minimal shrinkage.
The formulations for SMC compounds are well known to the skilled artisan. Generally the polymer is selected according to the desired properties of the product produced therefrom, as well as the processing parameters. In fabricating items from SMC, usually, the SMC resin is processed into a mat or pre-form and, then, placed under heat and several tons of pressure in a suitable mold to form the item, although other manufacturing processes may exist. The mold conforms the mat to the requisite part or shape. After forming the mat into the desired shape, the item produced is removed from the mold and it is, then, finished such as by painting or the like.
Where SMC is used for the manufacture of large items, such as automotive body panels, the finish is, preferably, applied through electrostatic spraying. However, other spray application methods may be used, such as, for example, siphon feed spray guns, gravity feed spray guns, High Volume Low Pressure (HVLP) spray guns, airless spray systems, air assisted airless spray systems, rotary atomizer, disk rotators and the like, as well as combinations thereof.
However, SMC panels, when exposed to electrostatic spray finishes, present certain unique problems. First, because SMC is a non-conductive plastic, it does not have the capacity to carry an electrical current. Therefore, a conductive solution or coating, such as a conductive primer, conductive preparation coat, conductive undercoat or the like must be applied to the SMC surface so that subsequent coatings can be applied thereover via electrostatic application.
Such conductive solutions or coatings are known in the prior art. Ordinarily, these prior art primers are either solvent-based, water based, powder coatings, or combinations thereof These primers, generally, require a cure at elevated temperatures in a suitable oven after application. However, because of the inherent nature of SMC, this creates certain problems. SMC materials are porous. When solvent evaporates or is removed from the coating, there is a tendency for surface irregularities, which are manifested by bubbly looking voids or xe2x80x9cpopsxe2x80x9d to appear or be created on the surface of the finished product. Oftentimes, the xe2x80x9cpopsxe2x80x9d may not be noticed until a final topcoat is applied and cured. Obviously, this causes a great expense in labor and materials to correct the problem. The porous nature of the SMC and the problems it creates vis-à-vis finishing, may be more pronounced where there are sharp edges associated with the molded part. Sharper edges in an SMC part may exhibit a greater propensity for the xe2x80x9cpopsxe2x80x9d to occur. Although manufacturing or molding techniques have been developed to deter the problems associated with the porosity, it still remains a problem. Presently, the techniques used to limit porosity vary from manufacturer to manufacturer. Among the most common practices used today are either air-drying or the use of force air-dried two part coating compositions. In either instance, though, this involves added labor, energy, opportunity and material costs, as well as volatile organic component (VOC) emissions.
Other manufacturers may abrade those areas which exhibit porosity, after the first coat of primer is applied and cured, by methods such as sanding those areas or the like. Thereafter, a second coat of primer is applied to the product. Clearly, this adds labor and material, as well as energy costs, for smoothing out the surface and removing the pops.
Aside from the irregularities, prior art coatings create added expense in their very application. Usually, with solvent-based, water-based, powder coating applications, and combinations thereof, a primer coating or multiple primer coatings is applied to the part, and may be followed by sealant coating(s), and, then, a topcoat, being applied thereover. Most of the above coating systems are cured at elevated temperatures. The topcoat is generally the last coating operation done in the system. The application of these layers can add expense in labor, VOC emissions, energy, and materials. The subsequent result is a reduced production rate.
As is detailed hereinafter, the present invention alleviates many of the problems encountered heretofore by providing a substantially 100% solids, sprayable, radiation curable, conductive coating composition which not only functions as a primer but may also be utilized as a sealer or a combination, as well.
The composition hereof, generally, comprises a mixture of: (a) a radiation curable, polymerizable compound which may be either a polymer, a pre-polymer, a monomer, and mixtures thereof; (b) a photoinitiator which may be a blend of photoinitiators; and (c) a conductive pigment or a mixture of pigments.
More particularly, the composition hereof, generally, comprises (a) from about 20% to about 99%, by weight, based on the total weight of the composition of a radiation curable, polymerizable compound, (b) from about 0.5% to about 18%, by weight, based upon the total weight of the composition, of the photoinitiator and (c) from about 0.5% to about 50%, by weight, based upon the total weight of the composition, of the electroconductive pigment.
The pigment mixture may comprise both conductive and non-conductive pigments in admixture including (a) mixtures of conductive pigments and (b) mixtures of conductive and non-conductive pigment.
Also the composition hereof may contain adjuvants such as flow additives, dispersion aids, defoamers, deaerators, suspension aids, and mixtures thereof. Other adjuvants, as needed, may also, be incorporated hereinto.
The composition may also contain fillers or extenders that may be organic or inorganic, as well as mixtures thereof.
The composition hereof is, preferably, used to coat SMC surfaces and, in particular, automotive panels. It is to be understood, however, that the present invention also contemplates coating SMC surfaces other than automotive panels, as well as other substrates such as, but not limited to, reaction injection molding (RIM), reaction thermal molding (RTM), hand lay up, acrylonitrile butadiene styrene (ABS), thermoplastic olefin (TPO), polycarbonate (PC), hybrid molding, and the like.
The present coating composition functions not only as a primer but as a sealer as well, thus, eliminating the need for separate primer and sealer coatings.
For a more complete understanding of the present invention, reference is made to the following detailed description and accompanying example.